doc: switch links from deprecated SLA dashboard to Grafana dashboard

Why is this change being made?

The GitLab SLA dashboard is being deprecated (gitlab-org/gitlab#346541 (closed)). We have the same information in our Grafana dashboards. This MR switches the links from the deprecated dashboards over to the Grafana ones.

Side note: the GitLab SLA dashboard is not currently working for me, while the Grafana one is.

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